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PTYS 554
Evolution of Planetary Surfaces
Volcanism IVolcanism I
PYTS 554 – Volcanism I 2
Volcanism I Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption
Volcanism II Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing
Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits
PYTS 554 – Volcanism I 3
Volcanoes on all the terrestrial bodies (and then some…)
Mercury – Smooth plains Moon – Maria Venus – Maat Mons
Earth – Mount Augustine Mars – Olympus MonsIo – just about everywhere
PYTS 554 – Volcanism I 4
Volcanism on Earth Mostly related to plate tectonics Mostly unseen. ~30 km3 per year (~90%) never reaches the surface
Rift-zone and subduction-zone volcanism has very different causes
PYTS 554 – Volcanism I 5
Volcanic material derived from the mantle Silicate composition built from SiO4 tetrahedra
Mantle rocks built from Olivine and Pyroxene
Olivine Isolated tetrahedra joined by cations (Mg, Fe) (Mg,Fe)2SiO4 (forsterite, fayalite)
Pyroxene Chains of tetrahedra sharing O atoms (Mg,Fe) SiO3 (orthopyroxenes)
(Ca, Mg, Fe) SiO3 (clinopyroxenes)
PYTS 554 – Volcanism I 6
Partial melting Rocks (incl. mantle rocks) are messy mixtures of many minerals In a pyroxene-olivine mixture the pyroxene melts more readily than the olivine More silica-rich minerals melt even more readily Melting mantle at the Eutectic has a specific composition – generally basaltic
PYTS 554 – Volcanism I 7
Magma is characterized by silica and alkali metal content
Partial melting of fertile mantle produces basalts
Higher temperatures mean more Olivine is melted (lowers Si/O ratio)
Proportionally lower Silica in melt Proportionally more Iron etc…
Io volcanism probably ultramafic High-temp melting of Earth’s mantle in early
history produced Komatiite – primitive basalt
UltrabasicPrimative
Basic AcidicEvolved
Fe poorLight
Less-dense
Fe richDark
Dense
PYTS 554 – Volcanism I 8
Recall that for the geotherm rolls over when radiogenic isotopes are in the crust
Steady-state solution: T = T0 + (Q/k) z – (H/2k) z2
When dT/dz=0 then z = Q/H ~ 100 km H~0.75 μW m-3 Q~0.08 W m-2
Ordinarily mantle material would never melt
Three ways to get around this (ranked by importance) Lower the pressure by moving mantle material upwards Change the solidus location (adding water)
Important only on Earth
Raise the temperature (plumes melting the base of the crust)
PYTS 554 – Volcanism I 9
Mantle temperatures follow an adiabat α : Thermal expansion coefficient Cp : Heat capacity
Works out to only ~ 0.25-0.5 K/km Material rises and cools at this rate (i.e. not much) …but pressure drop is large
Material can cross the melting curve
Convection creates near isothermal mantle Temperature changes accommodated across
boundary layers Heat transport across boundary layer is
conductive Rates of cm/year
z
T
Lithosphere
ΔTh
δ<<h
Ignore the lithosphere/asthenosphere
boundary in this figure
Decompression melting
PYTS 554 – Volcanism I 10
Most important mechanism for rift zones Requires a thin lithosphere Melting starts at ~50km
Mantle plumes can also create hot-spot volcanism with this mechanism Ocean island basalts
Accounts for ~75% of terrestrial volcanism …and probably 100% of planetary volcanism on other terrestrial planets
PYTS 554 – Volcanism I 11
Adding water changes the melting point As solid stability increases
Olivine – isolated tetrahedra Pyroxenes – chains Amphiboles – double chains Feldspar – sheets Quartz – 3D frameworks
Water breaks the Si-O bonds SiO2 + H2O -> 2 Si OH Acts in the same way that raising temperature does
Descending slabs loose volatiles From hydrated minerals e.g. mica at 100km From decomposition of marine limestones Causes mantle melting – leads to island arc basalts
Melosh, 2011
PYTS 554 – Volcanism I 12
Magma transport Mantle melt forms at crystal junctions
High surface energy
Wetting angle determines whether melt can form an interconnected network
<60° required for permeability
Less dense liquid flows upwards through the permeable mantle.
At mid-ocean ridges the asthenosphere comes all the way up to the base of the crust
Melt collects in magma chamber
PYTS 554 – Volcanism I 13
Lithosphere
Things are harder when there’s a lithosphere No partial melting (otherwise it wouldn’t be rigid) so no permeable flow Pressures at the base of the lithosphere are too high to have open conduits
Magma ascends through the lithosphere (and oceanic crust) in dikes Fine as long as ρ(magma) < ρ(country rock)
Magma ascends to the level of neutral bouyancy
Tilling and Dvorak, 1993
Magma
PYTS 554 – Volcanism I 14
What about under continents? Rising basaltic melt encounters continental
crust
Thin crust: basaltic volcanism still possible SW United states during Farallon subduction
Thick crust: Basalts don’t reach the surface Andes today Basalt underplates the crust and heats the continental
rock Melting produces felsic magma
Intermediate states are common so we have a wide variety of magma compositions in continental volcanism
Likewise for continental hotspot volcanism…
Under continental crust transport is harder Density change at the Moho Now ρ(magma) > ρ(country rock)
Magma chamber at the base of the crust
Felsic melts are still buoyant and can rise to form shallower magma chambers
PYTS 554 – Volcanism I 15
Differentiation occurs within magma chambers
Minerals condense and fall to the floor Cumulates
Follows Bowens reaction series
Melts become more felsic
Volatiles no longer kept in solution H2O and CO2
Starts to build pressure in the chamber
Pressure can force out magma – Eruptions!
Intrusive eruptions cool slowly below the surface
Extrusive eruptions cool quickly on the surface
Discontinuous Continuous
PYTS 554 – Volcanism I 16
Felsic magmas tend to have more water Water is a necessary component to form felsic melts and granites
PYTS 554 – Volcanism I 17
Intrusive structures Sills Dikes
PYTS 554 – Volcanism I 18
Intrusive structures Laccolith – bows up preexisting layers, so shallow Lopolith – subsidence from overlying layers - deep
PYTS 554 – Volcanism I 19
Batholith Many frozen magma chambers
PYTS 554 – Volcanism I 20
Formation of bubbles Reduces magma density – helps magma rise to the surface Also increases viscosity
Less water in the melt - Allows silica to polymerize Expanding bubbles cool magma
Emptying the magma chamber causes decompression More volatiles degassed – faster ascent etc… Leads to a ‘detonation front’ that propagates downwards Runaway effect until the magma chamber empties
Magma shredded by exploding bubbles If volatile content is very high If viscosity is very high and bubbles can’t escape Generates volcanic pumice and ash
PYTS 554 – Volcanism I 21
Volcanism I Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption
Volcanism II Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing
Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits
PYTS 554 – Volcanism I 22
Released volatiles power the eruption Injection of new magma Fractional crystallization Collapse of overburden Interaction with ground water Etc…